tations may be especially serious with respect to forest-dependent plant species.
These have a low proportional abundance in secondary successions for many
decades in any case (Finegan 1996), although under favorable conditions they
may accumulate in the understories of secondary forest (Guariguata et al.
1997). Conditions in shifting cultivation landscapes are far from optimal, how-
ever, and are especially hostile for this group of species, which therefore become
a priority for conservation action.
Discussion of factors that limit successional development leads us to
important specific cases of departure from the model presented at the begin-
ning of this section. Conditions in shifting cultivation patches may be so far
from the optimal for successional regrowth that they lead to the colonization
of the site by species that are capable of establishing dominance and inhibit-
ing successional change for many years. Among these are the grass Imperata
cylindrica(Whitmore 1984) and the shrub Chromolaena odorata(family Aster-
aceae; de Rouw 1995). The pantropical C. odorataoften is the dominant plant
in young fallows in Asia and West Africa (de Rouw 1995; Roder and Mani-
phone 1998; Kanmegne et al. 1999). De Rouw (1995) describes the condi-
tions under which the species may become dominant in the Côte d’Ivoire,
which we present as an example of how trends in shifting cultivation may lead
to the replacement of “normal” fallow vegetation by persistent lower-diversity
communities. Short fallows are the trigger. They are associated with greater
densities of arable weeds and lower densities of “forest plants” at the end of the
cropping period than is the case under traditional, longer fallows. This situa-
tion is in turn associated with slower canopy closure by the fallow vegetation
after cultivation ceases, which permits the establishment of C. odorata.The
tendency toward shorter fallows therefore is accompanied by a steady increase
in land area covered by C. odoratathickets (de Rouw 1995). This herb then
shades out other fallow species, becoming dominant. C. odorata-dominated
fallows, by definition, are of lower diversity than those free of this species.
Biodiversity and Conservation in Shifting
Cultivation Landscapes
The contribution of shifting cultivation landscapes to biodiversity conserva-
tion must be assessed in relation to two different human-made entities: crop
fields and fallows. Great care must also be taken regarding the approach to
assessment. Although biodiversity often is evaluated solely on the basis of
species richness, a full evaluation entails consideration of the composition of
the community and its ecological diversity (Finegan 1996). Biodiversity eval-
uations must also weight species, for example, according to whether they are
widely distributed or endemic, pioneers or forest-dependent (Pielou 1995).
Weighting is particularly important for agricultural communities and land-
scapes. This is because species richness may be high in such settings and could
166 III. The Biodiversity of Agroforestry Systems